Jitter measurement algorithm using locally in-order strobes

ABSTRACT

A method of jitter measurement is provided and includes sampling a device-under-test (DUT) output signal, having a repeating pattern, using an asynchronous clock over a desired period of time and mapping the samples onto a single period of the repeating pattern. Each period of the repeating pattern is sampled at least twice. A sampling frequency of the asynchronous clock is based on user inputs. Sampling the DUT signal comprises capturing logical state information representing each edge of a single period of the DUT signal at least once. The method further includes, separating the samples into subsets and mapping the sample subsets onto a single period of the repeating pattern wherein the samples within a particular subset are mapped to a set of times which are in the same order as in which the samples were obtained, processing the samples within each subset independently of samples in other subsets, and combining results of the processed subsets and processing the combined results of the subsets.

CROSS REFERENCE TO RELATED CASE

This application is related to U.S. patent application Ser. No.12/020,027, (referred to as the '2006 application), with a title“DETERMINING FREQUENCY COMPONENTS OF JITTER” filed on even dateherewith, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to signal testing and inparticular to a locally in-order strobing method for jitter measurementof a signal.

BACKGROUND

Automatic test equipment is commonly used by electronics devicemanufacturers for detecting manufacturing defects. For example,automatic test equipment allows semiconductor device manufacturers totest, on a large volume basis, the functionality of each device sold inthe marketplace. The tester drives signals to and detects signals from adevice-under-test (DUT) and evaluates the detected results to expectedvalues. Timing jitter degrades electrical systems and the push to higherdata rates and lower logic swings has increased interest and necessityfor the measurement and characterization of jitter.

Jitter is a key performance factor in high-speed data communications.Jitter is defined as the misalignment of the significant edges in asequence of data bits from their ideal positions. Misalignments canresult in data errors. Tracking these errors over an extended period oftime determines system stability. Jitter can be due to deterministic andrandom phenomena. Determining the level of these jitter componentsguides design improvement.

Jitter measurement techniques typically have the ability to measure thetiming of significant edges in a data stream. For example, oscilloscopesand digitizers have been used to measure the voltage of a signal atfixed time intervals and to analyze this data to determine edge times.Other examples are time interval analyzers and time stampers. Thesedevices directly measure edge times or the time between a pair of edges.In yet another example, asynchronous strobing comparator techniques areused to measure whether a signal is above or below a threshold at fixedtime intervals. Asynchronous strobing comparator techniques usestochastic mathematical techniques on the measurement data to determinecharacteristics of the edge time. Today there exist two general methodsof asynchronous comparator techniques, “In-Order” and “Out-of-order”strobing techniques. Shortcomings exist for both the in-order andout-of-order strobing techniques. For example in-order strobing has lownoise immunity and long acquisition times and out-of-order strobing issensitive to frequency errors, requires a complex setup for measurementand allows limited ability to analyze the frequency characteristics ofthe signal.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art forimproved strobing techniques.

SUMMARY

The above-mentioned problems as well as other problems are addressed byembodiments of the present invention and will be understood by readingand studying the following description.

A method of jitter measurement is provided. The method includes samplinga device-under-test output signal using an asynchronous sampling clockover a desired period of time. The DUT output signal has a repeatingpattern and each period of the repeating pattern is sampled at leasttwice. A sampling frequency of the asynchronous sampling clock is basedon user inputs. Sampling the DUT output signal comprises capturinglogical state information representing each edge of a single period ofthe DUT output signal at least once. The method further includes mappingthe samples onto a single period of the repeating pattern. The methodfurther includes separating the samples into one or more sample subsetswherein each sample subset contains samples that have effectively walkedacross the DUT output signal in order, processing the samples withineach subset independently of samples in other subsets and combiningresults of the processed subsets and processing the combined results ofthe subsets.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more easily understood and furtheradvantages and uses thereof more readily apparent, when considered inview of the description of the preferred embodiments and the followingfigures in which:

FIG. 1 a is a graphical illustration of one embodiment of signal strobemeasurement using a walking strobe according to the teachings of theprior art.

FIG. 1 b is a graphical illustration of an in-order strobing of thesignal strobe measurement of FIG. 1( a) according to the teachings ofthe prior art.

FIG. 2 a is a graphical illustration of one embodiment of signal strobemeasurement using a walking strobe (locally-in-order strobing) accordingto the teachings of the present invention.

FIG. 2 b is a graphical illustration of a locally-in-order strobing ofthe signal strobe measurement of FIG. 2( a) in one embodiment of thepresent invention.

FIG. 3 a is a graphical illustration of another embodiment of a signalstrobe measurement using a walking strobe (locally-in-order strobing)according to the present invention.

FIG. 3 b is a graphical illustration of a locally-in-order strobing ofthe signal measurement of FIG. 3( a) in one embodiment of the presentinvention.

FIG. 4 is a block diagram of one embodiment of a system for strobingsignals according to the teachings of the present invention.

FIG. 5 a is a flow chart for one embodiment of a method of locallyin-order strobing according to the teachings of the present invention.

FIG. 5 b is a flow cart illustrating the processing information from alocal in-order strobing of one embodiment of the present invention.

FIG. 6 is a flow chart for one embodiment of a method of locallyin-order strobing according to the teachings of the present invention.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize specific features relevantto the present invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments in which theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense.

Embodiments of the present invention provide methods and systems forimproved signal strobing techniques. In one embodiment, the signalstrobing techniques are used for performing jitter measurements withreduced acquisition times over current strobing techniques. Embodimentsof the present invention provide systems and methods to overcome shortcomings of current strobing techniques and improved frequency analysis.

Embodiments of the present invention are based on asynchronous strobingtechniques. In one embodiment, an asynchronous strobing method uses astrobe that appears to walk across an output signal of adevice-under-test (DUT). The DUT signal waveform has a repeating patternand a sample is taken every time Ts over a repeating pattern of thesignal waveform. Ts=(M×Tpat+Tres)/N, where N is an integer>1 and M is aninteger≧1. Tpat is the period of time for a single pattern of the DUTwaveform to occur. Tres is the effective sampling resolution.

The methods of undersampling are described as “Walking Strobe” becausean asynchronous clock is used to describe characteristics of a signalunder test. The signal under test must have a repetitive pattern. Theasynchronous nature of the strobe causes it to appear to walk across thetest signal. As a result, even though the actual samples will be takenover a time period many times larger than that of the test pattern (Tpat), they can be mapped to “effective times” all within Tpat. Thesamples are analyzed in M sub-sequences called “Strobe Subsets.” Oncemapped onto a single period Tpat of the DUT waveform, the strobes for agiven strobe subset are locally in-order. Locally in-order means thatthe strobes within a particular subset can be mapped to a set of timeswhich are spaced by Tres and in the same order as in which the strobeswere obtained. This results in a number of strobe subsets each comprisedof locally-in-order strobes containing logical state information about aportion of the DUT output waveform. This aquired data is used todetermine characteristics of the DUT output waveform such as randomjitter, data dependent jitter and total jitter.

FIG. 1 a is a graphical representation of an undersampled DUT outputsignal, shown generally at 100. Graph 100 includes a representation ofDUT signal 102 being sampled at time intervals Ts by strobes 106 ₀through 106 ₁₉ over twenty periods of Tpat 104. Due to space issues onthe page that FIG. 1 a is illustrated, it is noted that only 5 of the 20periods of Tpat are shown in FIG. 1 a. In this embodiment, it takes20×Ts to acquire the data and Ts=(Tpat+Tres).

FIG. 1 b is a graphical representation of in-order strobing, and each ofthe 20 periods of DUT signal 102 and strobes 106 ₀ through 106 ₁₉ fromFIG. 1 a are overlaid on top of one another, respectively. A singleperiod Tpat of DUT signal 120 is shown expanded with each strobe 106 ₀through 106 ₁₉ spaced at Tres.

FIG. 2 a is a graphical representation of an undersampled DUT signal,shown generally at 200, according to one embodiment of the presentinvention. A locally-in-order method of undersampling is applied and a“walking strobe” is utilized. In this embodiment, the acquisition ofdata is four times as fast as the acquisition of data for the DUT shownin FIG. 1 a. Graph 200 includes a representation of a DUT signal 202being sampled at time intervals Ts by strobes 206 ₀ through 206 ₁₉ overfive periods of Tpat 204. In this embodiment, it also takes 20×Ts toacquire the data but Ts is now 4 times shorter where Ts=(Tpat+Tres)/4.As a result, it only requires 5×(Tpat+Tres) to acquire the data versus20×(Tpat+Tres) for the acquisition of data with respect to FIG. 1 a.

FIG. 2 b is a graphical representation of the five periods of DUT signal202 from FIG. 2 a overlaid on top of one another, respectively. A singleperiod Tpat 204 of DUT signal 202 is shown with each strobe 206 ₀through 219 ₁₉ spaced at approximately Tres. In this embodiment, subsetsof strobes are grouped for further analysis. In this example, foursubsets of 5 strobes are defined. Subset 1 (SS₁) includes samples 206 ₀,206 ₄, 206 ₈, 206 ₁₂ and 206 ₁₆, from sample numbers 0, 4, 8, 12 and 16,subset 2 (SS₂) includes samples 206 ₁, 206 ₅, 206 ₉, 206 ₁₃ and 206 ₁₇from sample numbers 1, 5, 9, 13 and 17, subset 3 (SS₃) includes samples206 ₂, 206 ₆, 206 ₁₀, 206 ₁₄ and 206 ₁₈ from sample numbers 2, 6, 10, 14and 18 and subset 4 (SS₄) includes samples 206 ₃, 206 ₇, 206 ₁₁, 206 ₁₅and 206 ₁₉ from sample numbers 3, 7, 11, 15 and 19. Each subset SS₁-SS₄contains samples that are effectively, locally in-order and are eachseparated by Tres. Moreover, the total acquisition time of thelocally-in-order strobing (i.e. the time it took to acquire all the datagathered for the locally-in-order strobing) is close to the totalacquisition time to strobe the five periods of the DUT signal since thestrobes in the subsets SS1, SS2, SS3 and SS4 interleave each otherthroughout the five periods required to sample the DUT signal.

In one embodiment, each strobe subset SS₁-SS₄ overlaps an adjacentstrobe subset SS₁-SS₄. For example, samples of subset SS₂ includelogical state information representing portions of the DUT signal 202that overlap samples of adjacent subsets SS₁ and/or SS₃. In oneembodiment, each strobe subset SS₁-SS₄ includes samples containinglogical state information representing at least one edge within a singleperiod Tpat of the DUT signal 202.

A method of sampling is provided where locally in-order strobes aretaken for each repetition of test pattern Tpat. The strobes of eachstrobe subset SS₁-SS₄ appear to walk across a portion of the pattern. Inone embodiment, locally in-order strobing of FIGS. 2 a and 2 b isapplicable to jitter measurement and provides advantages over currentmethods of strobing for jitter measurement. For example the locallyin-order strobing method as described above reduces the time to acquirestrobing samples. Typically, the amount of low frequency noise detectedby a measurement increases as the time to perform that measurementincreases. Since locally in-order sampling generally takes lessmeasurement time than in-order sampling, it is less susceptible to lowfrequency noise.

Since the strobes for a given strobe subset are locally in-order, theyare not subject to many of the problems associated with out-of-ordersampling. The acquisition takes the reduced time of out-of-ordersampling as more than one strobe is taken for each repetition of thetest pattern. Each strobe subset is analyzed in-order and the resultsare combined to provide results for the entire pattern. Using thelocally in-order strobing and analysis techniques achieves the benefitsof in-order and out-of-order strobing techniques without problemsindicative of either technique.

FIGS. 2 a and 2 b are meant for illustration and it is understood thatany number of strobes may be utilized to obtain logical stateinformation representing the DUT output signal 202 or portions thereof.

In some embodiments of the present invention, acquisition time is eitherincreased or decreased to change the frequency content of themeasurement.

FIG. 3 a is a graphical illustration of another embodiment of anundersampled DUT waveform, shown generally at 300, according to thepresent invention. A locally-in-order method of undersampling is appliedand a “walking strobe” is utilized. In this embodiment, the acquisitionof data includes three times as many samples as the acquisition of datafor the DUT shown in FIG. 2 a.

FIG. 3 b is a graphical representation of five periods of DUT signal 302from FIG. 3 a overlaid on top of one another, respectively. A singleperiod Tpat of DUT signal 302 is shown with each strobe 306 ₀ through319 ₁₉ spaced at approximately Tres. In this embodiment, logical stateinformation representing each edge of DUT waveform 302 has been capturedby three different sample regions. For example, strobes 306 ₃₂ and 306₃₆ in sample region 1 (310), strobes 306 ₁₃ and 306 ₁₇ in sample region2 (311), and strobes 306 ₅₅ and 306 ₅₉ in sample region 4 (313) capturelogical state information representing part of unit interval (UI) 3 andas a result, the effective edge within unit interval 3 has been capturedby three sample regions.

Extra samples taken in each sample region 310-313 improves themeasurement three ways. First, the overlap in sample regions reduces therisk that an effective edge occurs at the intersection of two sampleregions and is consequently not captured by either. Second, theincreased number of samples reduces the effect of errors in a fewsamples. Third, as long as more than one instance of a pattern edge iscaptured, the measured time of the pattern (Tpat) can be compared to thepattern's expected time. The ratio between measured and expected is usedto reduce errors caused by strobe frequency errors and DUT drift.

The method of locally in-order sampling is adaptable to capture logicalstate information (high or low) of transition regions and/or unitintervals as many times as desired or needed by the user for analysis.It is understood that although FIG. 3 b is illustrated with overlappingsample regions that capture logical state information about each unitinterval of Tpat three times that any amount of capture is possible. Inone embodiment, logical state information about each edge of Tpat iscaptured only once. In one embodiment, each sample region includeslogical state information about an associated transition region plus aportion of the bounding stability regions of the transition region. Inone embodiment, the sample region overlap is sufficient to capturelogical state information about approximately 0.25 unit intervals ofstability regions on each side of the transition region. In someembodiments of the present invention, acquisition time is decreased toreduce the time required to perform the measurement at the expense ofimproved accuracy.

FIG. 4 is a block diagram of a locally in-order strobing system, showngenerally at 400, according to one embodiment of the present invention.System 400 includes automated test equipment 402 that receives one ormore output signals from a device-under-test 404 and provides an output405 to a processor 406 for analysis. In one embodiment, ATE 402 includesbuffer 418 that feeds a buffered DUT output signal 401 to comparator416. In one embodiment, buffer 418 is a differential buffer and receivesdifferential data from DUT 404 via output signals 401 and 411. The oneor more output signals are buffered and the output buffered signal iscompared to a reference signal V_(T) (an expected signal) to determineif the DUT performed as expected. In one embodiment, V_(T) is athreshold representing the edge transition voltage. An output ofcomparator 416 is representative of the logical state of the DUT'soutput waveform and is fed into latch circuit 414 for sampling of thewaveform based on sampling clock 407 produced by source clock 420.Sampling clock 407 is fed to counter 412 and latch 414 and samples ofthe DUT waveform are obtained. The samples are stored in memory device410 for transmission to processor 406 for further analysis.

In one embodiment, clock source 420 is programmed to strobe the DUTwaveform based on device specification and user inputs. User inputs arespecific to the DUT and the desired signal information required. In oneembodiment, ATE 402 further includes a user input device 450 that iseither integral or remotely coupled to ATE 402. In one embodiment,software to perform locally in-order strobing resides within system 400and utilizes user inputs received from user input device 450 tocalculate and setup system 400 for jitter characterization. In oneembodiment, user inputs include one or more of the bit period or unitinterval of the jitter measure pins, jitter measure pins, the number ofbits per pattern, the target effective sampling resolution and thenumber of pattern repetitions. The jitter measure pins indicate whichDUT transmit pins will be measured. The bits per pattern are the numberof bits of the repeating pattern Tpat. The target effective samplingresolution (Tres) determines the effective sampling frequency. Thenumber of pattern repetitions indicates how many times to effectivelywalk across the repeating DUT waveform pattern.

In one embodiment, processor 406 uses the logical state informationrepresentative of portions of the DUT output waveform, the user inputsand device data to calculate random jitter Rj of the DUT waveform. Inone embodiment, user inputs received from user input device 450 furtherinclude parameters which indicate measurement bandwidths. In oneembodiment, software to perform locally in-order strobing resides withinsystem 400 and utilizes user inputs received from user input device 450to calculate and setup system 400 for multiple jitter captures, eachwith a different bandwidth. In one embodiment, processor 406 uses thesampling strobe subsets/regions to perform eye measurement. The locallyin-order strobing method is used and multiple captures are performed atdifferent Vod level settings to create an eye diagram. In oneembodiment, the Vod level settings are at 20, 50 and 80 percent.

In operation, the DUT output signal(s) 401 and 411 have a repeatingpattern and are sampled using a walking strobe. The clock period ofsampling clock 407 is set at a fraction of a time greater than theperiod of the repeating pattern so that each clock samples the patterneffectively the fraction of time later or earlier than a previoussample. In one embodiment, for example, the fraction of time is apicosecond and the period of the repeating pattern output signal is amillisecond.

FIG. 5 a is a flow chart for a locally in-order strobing technique,shown generally at 500, according to one embodiment of the presentinvention. The method comprises asynchronously digitally undersampling adevice-under-test (DUT) output waveform. The method begins at 502 andreceives one or more DUT waveforms. The DUT waveform has a repeatingpattern. Based on user inputs, the method strobes the DUT waveform usingan asynchronous clock at 504 using a locally in-order strobing scheme.The method proceeds to 506 and obtains samples of the signal over adesired period. The asynchronous nature of the strobes causes them toappear to walk across the DUT waveform. The method obtains more than onesample per period Tpat of the DUT waveform. Each sample is part of asample subset with samples that are taken at a desired period of(M×Tpat+Tres) wherein Tres is a fraction of Tpat and M is an integer≧1.The method proceeds to 508 and stores digital representations of thesamples. The method proceeds to 510 and processes the stored samples.

In one embodiment, the locally in-order strobing scheme comprisesstrobing each edge of one period Tpat of the DUT waveform at least onceand including a portion of stable regions bounding each edge. Thelocally in-order strobing scheme is based on user inputs. In oneembodiment, user inputs include one or more of the bit period or unitinterval of the jitter measure pins, jitter measure pins, the number ofbits per pattern, the target effective sampling resolution Tres and thenumber of pattern repetitions. The jitter measure pins indicate whichDUT transmit pins will be measured. The bits per pattern is the numberof bits of the repeating pattern Tpat. The target effective samplingresolution (Tres) determines the effective sampling frequency. Thenumber of pattern repetitions indicates how many times to effectivelywalk across the repeating DUT waveform pattern.

In one embodiment, processing the stored samples includes mapping thesamples to “effective times” within a single period Tpat of the DUT.Once the samples are remapped each sample within an associated sampleregion is approximately separated from an adjacent sample by Tres. Inone embodiment, processing includes calculating jitter that includes oneor more of random jitter, deterministic jitter and total jitter. In oneembodiment, to analyze jitter, each sample region will be analyzedseparately to find all average edge positions occurring within a sampleregion/subset.

Furthermore the processing of the stored samples in embodiments of thepresent invention is further illustrated in the flow chart, showngenerally at 550, of FIG. 5 b. As illustrated, the samples at 552 areseparated into subsets. At 554, the samples are mapped into a singleperiod of a repeated pattern. In particular, samples within a particularsubset are mapped to a set of times which are in the same order as inwhich the samples were obtained. The samples in each subset are thenprocessed separately from the samples in other subsets at 556. Theresults of each processed subset, at 558, are combined. The combinedresults are then processed at 560.

In one embodiment, the method of locally in-order strobing is used todetermine if characteristics of the DUT waveform conform to voltageversus time requirements of the DUT. These requirements may consist ofrisetime, falltime, or template. In this embodiment, thelocally-in-order strobing is repeated a plurality of times, resulting inmultiple segments of waveform data. Each time, the DUT output signal iscompared to a reference signal V_(T) (an expected signal) set to adifferent voltage. The waveform segments may be adjacent ones or morewidely spaced samples of the DUT signal. In another embodiment, the DUTis strobed at multiple levels at the same time by duplicating thecircuitry. In operation, the multiple segments of waveform data areprocessed to determine characteristics such as earliest, latest, andaverage edge positions at each voltage level. The average positions ofan edge at two voltage levels can be used to determine that edge'srisetime or falltime. The earliest and latest times that each edgeoccurs can be used to determine whether the DUT output waveform's edgesfit within a required template or eye diagram.

In one embodiment, the locally in-order strobing scheme comprisesstrobing each edge of one period Tpat of the DUT waveform at least onceand including a portion of stable regions bounding each edge. The methodobtains more than one sample per period Tpat of the DUT waveform. Eachsample is part of a sample subset with samples that are taken at adesired period of Tpat×M+Tres wherein Tres is a fraction of Tpat and Mis an integer≧1.

In one embodiment, processing the stored samples includes mapping thesamples to “effective times” within a single period Tpat of the DUT.Once the samples are mapped each sample is separated from an adjacentsample by Tres. In one embodiment, processing includes calculatingjitter that includes one or more of random jitter, data dependentjitter, deterministic jitter and total jitter. In one embodiment, toanalyze jitter, each sample region will be analyzed separately to findall average edge positions occurring within a sample region/subset.

FIG. 6 is a flow chart for determining jitter in a waveform, showngenerally at 600, according to one embodiment of the present invention.First at 602 the data within a sample region is looped through onesample at a time, starting at the first bit sampled and ending at thelast bit sampled. Most of the data will consist of logical Lows or Highssurrounded by data of the same state. At 604, transition regions withinthe data are identified. The transition regions are identified as a setof data containing a mixture of zeros and ones like the example in FIG.2 d of the '2006 application incorporated herein. Generally, a rule isneeded to define what separates one transition region from another. Forexample, when the data in a waveform contains the equivalent of unitinterval (UI)/4 of steady state, any different data will be consideredthe start of a new transition region. Moreover, a general rule is alsoneeded to ignore data that may indicate a transition region at thebeginning or the end of a sample region.

Since each transition region indicates an apparent edge, at 606calculations are performed to determine the standard deviation of eachtransition region. For practical purposes, the standard deviation is therandom jitter although it may contain negligible amounts of periodjitter and the like. At 606, calculations are also performed todetermine the mean position of each apparent edge, as well as theearliest and latest times that apparent edge occurred. The methodproceeds to 608 and determines if the last data in the last region hasbeen looped through. If yes, the method proceeds to 610. If the lastdata in the last region has not been looped through, the method proceedsto 602 and begins again.

The method proceeds to 610 where each apparent edge in each sampleregion is associated with an edge in the data pattern. Generally, a ruleis needed because edges do not occur at their ideal times. For example,if the first apparent edge found is considered to be at time 0 andassociated with pattern edge 0, another apparent edge may be associatedwith pattern edge N when that edge effectively occurs within the rangeN(UI)+/−0.5UI.

Drift and/or frequency differences may occur between the expected datarate and the DUT's data rate. Errors may also exist in the samplingrate. These issues are dealt within steps 612-620 of flow chart 600.Drift and/or frequency differences manifest themselves as an error thatincreases at a constant rate through the acquisition of the measurement.Therefore, if the rate at which the errors increase can be determined asa function of time, the error can be removed from each apparent edge'sposition by calculating the error associated with the time at which thatedge was acquired. At 612-616 the measured UI is calculated for eachpattern edge based on all the apparent edges associated with thatpattern edge. In one embodiment, step 614 is performed using a leastsquares linear regression methodology. The method proceeds to 618 wherethe measured UI of all pattern edges is calculated as the average of themeasured UIs calculated for each pattern edge. The rate at which theerrors increase can be determined by the ratio between the expected UIand the measured UI. The method proceeds to 620 and the mean position ofeach apparent edge is adjusted based on the determined ratio and thenumber of UIs that occurred between the beginning of the acquisition andthe time that apparent edge was acquired. Locally in-order strobing isunlike out-of-order sampling in that out-of-order sampling acquires theinformation to determine an apparent edge's mean position at a range oftimes spread throughout the measurement. When data from very differentacquisition times is merged to generate the mean effective position ofan apparent edge, as is the case with out-of-order samplingcalculations, it is not possible to determine a time at which that edgewas acquired. Consequently, the rate at which errors increase canneither be established nor applied to apparent edge positions.

The method proceeds to 622-628 and determines the average position anderror of each pattern edge. At 624, the average position of each patternedge is calculated as the average position of all the apparent edgesassociated with that pattern edge. At 626 each pattern edge's error iscalculated as the difference between the edge's mean effective positionand its ideal position. The method proceeds to 630 and computes the DataDependent jitter as the difference between the most positive patternedge error and the most negative pattern edge error.

Embodiments of the present invention provide systems and methods oflocally in-order strobing that has a fast sampling frequency and as aresult minimizes the acquisition time and rejects low-frequency jittersuch as periodic jitter.

As stated above, the methods and techniques described here areimplemented by a locally in-order strobing system. Embodiments ofdevices that make up the locally in-order strobing system may beimplemented in digital electronic circuitry, or with a programmableprocessor (for example, a special-purpose processor or a general-purposeprocesser such as a computer firmware, software, or in combinations ofthem). Apparatus embodying these techniques may include appropriateinput and output devices, a programmable processor, and a storage mediumtangibly embodying program instructions for execution by theprogrammable processor. A process embodying these techniques may beperformed by a programmable processor executing a program ofinstructions to perform desired functions by operating on input data andgenerating appropriate output. The techniques may be implemented in oneor more programs that are executable on a programmable system includingat least one programmable processor coupled to receive data andinstructions from, and to transmit data and instructions to, a datastorage system, at least one input device, and at least one outputdevice. Generally, a processor will receive instructions and data from aread-only memory and/or a random access memory. Storage devices suitablefor tangibly embodying computer program instructions and data includeall forms of non-volatile memory, including by way of examplesemiconductor memory devices, such as EPROM, EEPROM, and flash memorydevices; magnetic disks such as internal hard disks and removable disks;magneto-optical disks; and CD-ROM disks. Any of the foregoing may besupplemented by, or incorporated in, specially-designedapplication-specific integrated circuits (ASICs).

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiments shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is intended that this invention be limited onlyby the claims and the equivalents thereof.

1. A method of locally in-order strobing, comprising: receiving at leastone device-under-test (DUT) output signal, wherein the DUT output signalhas a repeating pattern; sampling the DUT output signal using anasynchronous sampling clock, wherein each period of the repeatingpattern is sampled at least twice; wherein a sampling frequency of theasynchronous sampling clock is based on one or more user inputs; whereinsampling the DUT output signal comprises capturing logical stateinformation representing each edge of a single period of the DUT outputsignal at least once; separating the samples into subsets, a firstsubset comprising a first sample from each period of the repeatingpattern and a second subset comprising a second sample from each periodof the repeating pattern; mapping the samples onto a single period ofthe repeating pattern, wherein the first subset maps to a first portionof the single period and the second subset maps to a second portion ofthe single period, wherein the samples within a particular subset aremapped to a set of times which are in the same order as in which thesamples were obtained; processing the samples within each subsetindependently of samples in other subsets; combining results of theprocessed subsets; and processing the combined results of the subsets.2. The method of claim 1, wherein the one or more user inputs comprisesa number of bits per one period of the repeating pattern, a length of asingle bit period, a target effective sampling resolution, a number oftimes to effectively walk across the repeating pattern and jittermeasure pins to be sampled.
 3. The method of claim 1, wherein samplingthe DUT output signal further comprises capturing logical stateinformation representing each edge and a portion of adjacent stableregions at least once.
 4. The method of claim 1, wherein each samplewithin the single period of the DUT output signal belongs to a differentsample subset.
 5. The method of claim 1, further comprising storing thesamples in memory.
 6. The method of claim 1, further comprisingprocessing the stored samples.
 7. The method of claim 1, wherein aneffective position of samples of a single subset overlap samples ofadjacent subsets.
 8. The method of claim 1, further comprisingcalculating jitter of each edge based on samples of an associatedsubset.
 9. The method of claim 1, further comprising calculating one ormore of random jitter, data dependent jitter, and total jitter for theDUT output signal.
 10. The method of claim 9, further comprisingcalculating frequency components of the jitter for the DUT outputsignal.
 11. The method of claim 1, further comprising reducing the riskthat an effective edge occurs at an intersection of two sample subsetsand is not captured by either subset by increasing the number of sampleswithin each sample subset.
 12. The method of claim 1, further comprisingreducing the effect of errors in a few samples by increasing the numberof samples.
 13. The method of claim 1, further comprising reducingerrors caused by strobe frequency errors and DUT drift by determining arate at which errors increase as a function of time and applying thesedetermined errors as a factor affecting measurement results.
 14. Themethod of claim 1, further comprising reducing the time required toperform a measurement by decreasing the number of samples obtained. 15.A method of jitter measurement, comprising: sampling a device-under-test(DUT) output signal using an asynchronous sampling clock over a desiredperiod of time; wherein the DUT output signal has a repeating patternand each period of the repeating pattern is sampled at least twice;wherein a sampling frequency of the asynchronous sampling clock is basedon one or more user inputs; wherein sampling the DUT output signalcomprises capturing logical state information representing each edge ofa single period of the DUT output signal at least once; separating thesamples into subsets and mapping the samples onto a single period of therepeating pattern wherein the samples within a particular subset aremapped to a set of times which are in the same order as in which thesamples were obtained, wherein a first subset maps to a first portion ofthe single period and a second subset maps to a second portion of thesingle period; processing the samples within each subset independentlyof samples in other subsets; combining results of the processed subsets;and processing the combined results of the subsets.
 16. A method ofmeasuring jitter, comprising: receiving at least one device-under-test(DUT) output signal, wherein the DUT output signal has a repeatingpattern; receiving a plurality of user inputs; calculating a samplingfrequency based on the user inputs; sampling the DUT output signal basedon the calculated sampling frequency; mapping the samples onto a singleperiod of the repeating pattern wherein the samples are separated intosample subsets and the time period between each sample in a subset is afraction of time greater than a period of the repeating pattern so thateach sample of a subset is effectively a fraction of time later than theprevious sample after mapping the samples onto a single period of therepeating pattern; processing the samples within each subsetindependently of samples in other subsets; combining results of theprocessed subsets; and processing the combined results of the subsets.17. The method of claim 16, wherein the user inputs comprise one or moreof number of bits per one period of the repeating pattern, a length of asingle bit period, a target effective sampling resolution, a number oftimes to effectively walk across the repeating pattern and jittermeasure pins to be sampled.
 18. The method of claim 16, wherein eachsample subset includes at least one edge of a single period of the DUTsignal.
 19. The method of claim 16, wherein each sample subset includesat least one edge of a single period of the DUT signal and a portion ofstable regions bounding each edge.
 20. The method of claim 16, furthercomprising storing the samples in memory.
 21. The method of claim 20,further comprising processing the stored samples.
 22. The method ofclaim 16, further comprising calculating jitter of each edge based onsamples of an associated subset.
 23. The method of claim 16, furthercomprising calculating one or more of random jitter, data dependentjitter, and total jitter for the DUT output signal.
 24. The method ofclaim 16, further comprising determining if characteristics of the DUToutput signal conforms to voltage versus time requirements of thedevice-under-test.
 25. The method of claim 24, wherein thecharacteristics include one or more of an effective edge's risetime andfalltime.
 26. The method of claim 16, further comprising determiningwhether edges of the DUT output signal fit within a required template oreye diagram.
 27. An automated test equipment system, comprising: acomparator adapted to receive at least one output signal from adevice-under-test and compare the output signal to an expected outputsignal, wherein the output signal has a repeating pattern; a clocksource adapted to produce a sampling clock based on user inputs; and alatching circuit adapted to obtain samples of the output signalaccording to the sampling clock, wherein the samples are separated intosample subsets and the time period between each sample in a subset is afraction of time greater than a period of the repeating pattern so thateach sample of a subset is effectively a fraction of time later than theprevious sample after mapping the samples onto a single period of therepeating pattern; and a processor adapted to process samples withineach subset independently of samples in other subsets, the processorfurther adapted to combine the results of all the processed subsets andprocess the combined results.
 28. The system of claim 27, wherein theuser inputs comprise one or more of a number of bits per one period ofthe repeating pattern, a length of a single bit period, a targeteffective sampling resolution, a number of times to effectively walkacross the repeating pattern and jitter measure pins to be sampled. 29.The system of claim 27, further comprising a memory device adapted tostore the sampled data.
 30. The system of claim 29, further comprisingthe processor is adapted to analyze the stored data.
 31. The system ofclaim 30, wherein the processor maps the samples onto a single period ofthe repeating pattern.
 32. The system of claim 30, wherein the processorcalculates jitter of each edge of a period of the output signal based onsamples of an associated subset.
 33. The system of claim 27, whereinlogical state information representing each edge within a single periodof the output signal is captured at least once.
 34. The system of claim27, wherein logical state information representing each edge and aportion of adjacent stable regions within a single period of the outputsignal is captured at least once.
 35. The system of claim 27, whereineach sample within the single period of the output signal belongs to adifferent sample subset.
 36. The system of claim 30, wherein theprocessor calculates one or more of random jitter, data dependentjitter, and total jitter for the output signal.
 37. The system of claim30, wherein the processor determines if characteristics of the outputsignal conforms to voltage versus time requirements of thedevice-under-test, wherein the characteristics include one or more of aneffective edge's risetime and falltime.
 38. The system of claim 30,wherein the processor determines whether edges of the DUT output signalfit within a required template or eye diagram.
 39. An automated testequipment system, comprising: a buffer adapted to receive at least oneoutput signal from a device-under-test, wherein the output signal has arepeating pattern; a comparator coupled to an output of the buffer andadapted to compare the buffered output signal to an expected outputsignal; a clock source adapted to produce a sampling clock based on userinputs; a latching circuit adapted to obtain samples of an output signalof the comparator according to the sampling clock, wherein the samplesare separated into sample subsets and the time period between eachsample in a subset is a fraction of time greater than a period of therepeating pattern so that each sample of a subset is effectively afraction of time later than the previous after ma in the samples onto asingle period of the repeating pattern, and a processor adapted toprocess samples within each subset independently of samples in othersubsets, the processor further adapted to combine the results of all theprocessed subsets and process the combined results.